Field of the Invention
The present invention relates to a negative electrode active material for a lithium secondary battery having excellent capacity characteristics and cycle lifetime, and a lithium secondary battery comprising the same.
Description of the Related Art
Recently, in line with realization of miniaturization and weight reduction of electronic apparatuses and generalization of the use of portable electronic devices, research into lithium secondary batteries having high energy density as power sources thereof has been actively conducted.
Lithium secondary batteries are prepared by charging an organic electrolyte or a polymer electrolyte between a positive electrode and a negative electrode, and generate electrical energy by a redox reaction occurred when lithium ions are intercalated and deintercalated between the positive electrode and the negative electrode.
Since an organic electrolyte is used in a secondary battery, the secondary battery exhibits high energy density in which a discharge voltage thereof is two times or more than that of a typical battery using an alkaline aqueous solution.
Lithium transition metal oxides such as LiCoO2, LiMn2O4, and LiNi1-xCoxO2 (0<x<1), in which intercalation of lithium ions is possible, are mainly used as a positive electrode active material of the lithium secondary battery. Also, a material, which may reversibly receive and supply lithium ions while maintaining structural and electrical properties, is used as a negative electrode active material of the lithium secondary battery. For example, lithium, metal containing lithium, or a carbon-based material, such as natural graphite and hard carbon, which is almost similar to lithium in which intercalation/deintercalation of lithium ions is possible, are mainly used. At this time, a battery using lithium or an alloy thereof as a negative electrode active material is referred to as a lithium metal battery and a battery using a carbon material as a negative electrode active material is referred to as a lithium-ion battery.
Meanwhile, since charge capacity of an electrode using a carbon-based negative electrode active material is low at 360 mAh/g (theoretical value: 372 mAh/g), there may be limitations in providing a lithium secondary battery having excellent capacity characteristics.
Accordingly, an inorganic material-based active material, such as silicon (Si), germanium (Ge), antimony (Sb), or titanium (Ti), which may store/release lithium (Li) by an alloying reaction with lithium, has been studied as a new material that may replace the carbon-based negative electrode active material.
The inorganic material-based active material, in particular, a silicon-based negative electrode active material may have a relatively large amount of bound lithium (theoretical maximum: Li4.1Si) and this corresponds to a theoretical capacity of about 4200 mAh/g.
However, since the inorganic material-based negative electrode active material, such as silicon, may cause a large volume change during intercalation/deintercalation of lithium, i.e., charge and discharge of a battery, pulverization may occur. As a result, a phenomenon of agglomeration of pulverized particles may occur, and thus, the negative electrode active material may be electrically extracted from a current collector and this may cause loss of reversible capacity under a prolonged cycle. For example, a capacity of a lithium secondary battery using a silicon-based negative electrode active material may be similar to that of a battery using graphite after about 12 cycles. Therefore, a previously known inorganic material-based negative electrode active material, e.g., a silicon-based negative electrode active material, and a lithium secondary battery comprising the same may exhibit low cycle lifetime characteristics and capacity retention ratio despite of advantages according to their high charge capacity.
In order to address the foregoing limitations, there have been attempts to use a carbon and silicon-based nanoparticle composite as a negative electrode active material or use a negative electrode active material including carbon material and metal or semi-metal carbide coating layers (see Patent Document 1), a negative electrode active material including a coating layer including inorganic oxide particles on a surface of a core including lithium-vanadium-based oxide (see Patent Document 2), a negative electrode active material coated with a fluorine-based compound in the form of a complex salt (see Patent Document 3), and a negative electrode active material having an amorphous carbon layer formed on nanotubes containing a non-carbon-based material such as silicon. However, since the above negative electrode active materials may also exhibit a relatively large amount of loss of reversible capacity over a prolonged cycle, cycle lifetime characteristics and capacity retention ratio may not be sufficient. Also, capacity characteristics itself may not be sufficient due to the relatively large amount of carbon included in the nanocomposite.